It is well known that there is a need for providing wheel-slip detection apparatus on modern high speed transit vehicles and railway trains in order to safely and efficiently slow down and/or stop the vehicles or train at a station or the like. A slip occurs when more braking force is exerted on a wheel axle unit than that which can be sustained by the available amount of frictional adhesion that exists between the wheels and the rails. The slipping condition causes the axle to decelerate at a higher rate than the vehicle deceleration. The object of wheel-slip control is to reduce the braking force to a point where it is lower than the equivalent force of the available adhesion. This will stop the axle from decelerating faster than the vehicle and will cause the axle to accelerate back up to the speed of the vehicle. It will be appreciated that during the time when an axle is decelerating faster than the speed of the vehicle, a velocity difference is developed between the axle and the vehicle. Previously, most wheel-slip controllers attempted to anticipate when the axle would return to the speed of the vehicle and then would strive to nullify the brake force reduction at that point. In most cases, the wheel-slip controller would make this decision on the basis of when the highest velocity axle on the vehicle and the slipping axle reached a given speed differential and/or when the acceleration rate of the axle under control reached a certain preset value. Themajor problem with these previous wheel-slip controllers resides in the fact that the preset points of anticipation are not always the right values for all adhesion and speed conditions. Under certain conditions, the prior wheel-slip controllers were susceptible to losses in vehicular performance and, in some cases, resulted in a locked axle returned or when it was erroneously anticipated that the axle was back up to the speed of the vehicle.
Accordingly, it is an object of the invention to provide a new and improved rate polarity shift wheel-slip control system.
Another object of this invention is to provide a unique wheel-slip control system which utilizes a rate polarity shift to determine when a brake force reduction on a slipping axle should be cancelled.
A further object of this invention is to provide a novel wheel-slip control system which employs a positive-to-negative polarity shift for ascertaining when a slipping axle has ceased to slip and has returned to the speed of the vehicle.
Yet another object of this invention is to provide an improved vehicle brake control system which cancels a brake force reduction when the rotation of a slipping wheel has been restored to the velocity of the vehicle.
Yet a further object of this invention is to provide a wheel-slip control comprising, means for producing signals representative of the velocity of each of the wheels, means for differentiating the velocity signals to obtain rate signals, means for determining the most negative-going rate signal of each wheel axle unit of each truck, means responsive to the most negative-going rate signal to initiate a brake force reduction action on the truck experiencing a wheel-slip condition, and means for sensing a polarity shift in the most negative rate signal to cause the responsive means to reapply a braking action.